A predominant number of color picture tubes in use today have line screens and shadow masks that include slit-shaped apertures. The apertures are aligned in columns, and the adjacent apertures in each column are separated from each other by webs or tie bars in the mask. Such tie bars are essential in a the mask, to maintain its integrity when it is formed into a dome-shaped contour which somewhat parallels the contour of the interior of a viewing faceplate of the tube. Tie bars in one column are offset in the longitudinal direction of the column (vertical direction) from the tie bars in the immediately adjacent columns. Because of the pattern of apertures and tie bars, the upper and lower borders of the aperture array are somewhat jagged. Some aperture columns end with tie bars near the border, while other columns end with apertures at the border. When a viewing screen is formed using such a mask as a photomaster, the resultant screen also has jagged upper and lower borders. Such jagged borders are esthetically undesirable.
A technique that produces screens with smooth borders is disclosed in U.S. Pat. No. 4,300,070, issued to R. H. Godfrey et al. on Nov. 10, 1981. In that patent, the aperture array border is smoothed with a elaborate method of lengthening and shortening the pitches of the last two apertures in each column. The method made the border geometry smooth and also equalized the light output at the edges of the screen. However, the method was developed for use with masks having a constant tie bar pitch and a projected straight line border on the screen.
Another technique of eliminating jagged screen borders, for use with a mask having curved upper and lower borders, is disclosed in U.S. Pat. No. 4,631,440, issued to J. D. Robbins on Dec. 23, 1986. In that patent, the upper and lower borders are made smooth by varying the vertical pitch from column-to-column, while keeping the number of apertures per column and the pitch in each individual column constant.
Changes in the aperture pitch in a mask have an effect on a phenomenon called moire. When electron beams strike the shadow mask, the tie bars block portions of the beams, thus causing shadows on the screen immediately behind the tie bars. When the electron beams are repeatedly scanned in a direction perpendicular to the aperture columns (horizontal direction), they produce a series of bright and dark horizontal lines on the screen. These bright and dark horizontal lines interact with the shadows formed by the tie bars, creating lighter and darker areas and producing a wavy pattern on the screen, called a moire pattern. Such moire pattern greatly impairs the visible quality of images displayed on the screen. Therefore, when tie bar pitch is varied, it is highly desirable to select such a pitch that will minimize the moire pattern.
There have been many techniques suggested to reduce the moire problem. Most of these techniques involve rearranging the locations of the tie bars in a mask, to reduce the possibility of the electron beam scan lines beating with the tie bar shadows. Although many of these techniques have been used successfully to reduce moire, they are often incompatible with the desire to obtain smooth clean looking top and bottom screen edges. Therefore, there is yet a need for a shadow mask aperture array pattern that will provide smooth, clean-looking top and bottom screen edges, while simultaneously reducing moire patterns on a tube screen.